Many pourable food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of a sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing a laminated web packaging material. The laminated packaging material comprises layers of fibrous material, e.g. paper, covered on both sides with thermoplastic material, e.g. polyethylene. In the case of packages for long-storage products (such as UHT milk), the side of the packaging material eventually contacting the food product also has a layer of barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which in turn is covered with a layer of thermoplastic material.
As known, packages of this sort, provided or not with the layer of barrier material, are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution; once sterilization is completed, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packaging material.
The sterilized web of packaging material is maintained in a closed, sterile environment and is folded into a cylinder and sealed longitudinally to form a tube.
The tube is fed in a vertical direction parallel to its axis, and is filled continuously with the sterilized or sterile-processed food product.
The packaging unit interacts with the tube to heat seal equally spaced cross sections of the tube and form pillow packs connected to the tube by transverse sealing bands.
More specifically, the packaging unit comprises two forming assemblies movable along respective guides and which interact cyclically and successively with the tube to heat seal the packaging material of the tube.
Each forming assembly comprises a slide, movable back and forth along the respective guide, and two jaws hinged at the bottom to the respective slide and movable between a closed position, in which they cooperate with the tube to heat seal it at a cross section thereof, and an open position, in which they are detached from the tube.
More specifically, the jaws of each forming assembly are moved between the open and the closed position by respective cams.
The forming assemblies operate a half-period out of phase: one forming assembly moves up, with its jaws open, while the other forming assembly moves down, with its jaws closed, to avoid collision and interference between the assemblies.
The jaws of each forming assembly are fitted with respective sealing members cooperating on opposite sides with the tube, and defined, for example, by a heating member and by a counter-pressure member configured to provide mechanical support to grip the tube with the necessary pressure.
Each forming assembly also comprises a pair of forming members in the form of half-shells, which are hinged to the respective jaws and interact mutually to fold the tube between two consecutive sealed sections and define the volume of the package being formed.
The forming members of each forming assembly are movable cyclically between a first operating position, in which, in relation to the position of the respective jaws, the forming members have the maximum distance from one another, and a second operating position, in which, in relation to the closed position of the respective jaws, the forming members mate with one another and cooperate with the tube to define the shape and volume of the package being formed.
The forming members are usually spring-loaded elastically into the first operating position and have respective rollers which cooperate with relative cams designed to close the forming members about the tube when the relative forming assembly is in a predetermined position.
Though of excellent performance in general, packaging units of the type described above still leave room for further improvement.
In particular, at certain travelling speeds of the forming assemblies, the forming members reaching the first operating position produces shock, which dynamically stresses the packaging unit and which can only be reduced—by appropriately adjusting the stiffness of the springs and the mass of the moving parts—at a given travelling speed of the slide, thus limiting output and flexibility of the packaging unit.
To solve this kind of problem, EP-A-1795447 proposes a packaging unit also including retaining means configured to cooperate with the forming members of each forming assembly during their strokes towards the first operating position in order to oppose the elastic force acting on the forming members themselves and to reduce dynamic stress at the end of such strokes.
The retaining means of each forming assembly comprise a first member carried by the slide and two second members carried by the respective forming members and each cooperating in sliding manner with the first member to reduce the elastic thrust exerted on the forming members themselves during their strokes towards the first operating position.
In one embodiment disclosed in EP-A-1795447, the first member is defined by a piston of a variable-length actuator and the second members are defined by respective operating arms protruding from the respective forming members and cooperating in sliding manner with such piston.
In particular, interaction between the piston of the actuator and the operating arms is used to reduce the elastic force on the forming members during their strokes towards the first operating position as well as to cause the forming members to perform opposite strokes towards the second operating position.
Though offering a more flexible solution to the problems posed by the use of cams to control movement of the forming members, packaging units of the type disclosed in the above-described embodiment of EP-A-1795447 still leave room for further improvement.
In particular, due to the fact that in the open position of the jaws of each forming assembly, the relative actuator looses contact with the operating arms of the forming members, the latter have to be moved to the first operating position before the respective jaws leave the tube.
This necessarily imposes a constraint to the work cycle of the forming members.
Moreover, the entire upward movement of the slide of each forming assembly with the jaws opened (i.e. the return stroke) has to be performed also with the forming members in the first operating position, only kept there by the respective springs. This means that no control of the position of the forming members can be performed in the return stroke and that such forming members may be subjected to undesired vibrations and dynamic stresses.